Mechanisms for positioning the work spindles are required for program-controlled milling and drilling machines (machining centers) so as to permit an automatic change of tools by means of a correspondingly designed tool changing assembly. For permitting the grippers of the changing assembly to reliably grip the tool which is to be changed, the tool and the work spindle must be jointly moved to and fixed in a predetermined angular position.
Various designs of mechanisms for positioning the spindle are already known, but such mechanisms are of complex design and expensive to manufacture. In one such positioning mechanism known from German patent publication No. 3,632,106, a cardioid curve is formed on the quill which is fixedly connected to the spindle. In the spindle housing, a plunger is mounted for movement by an actuating drive, the plunger having a rolling body rolling along the cam surface formed by the cardioid curve. When the spindle drive is shut off, the plunger is advanced by its actuating drive prior to each tool changing operation, whereby a corresponding rotation of the work spindle, which is disengaged from its drive train, to a position determined by the cardioid curve, is brought about. In this position of the work spindle the tool changing operation takes place by means of an automatic tool changing assembly, the grippers of which engage the collar of the respective machine taper to withdraw the tool after release of the collet chuck of the spindle from the latter.
For fixing the adjusted spindle position an additional spring-loaded fixing bolt may be provided in the spindle housing, which engages in a bore of the quill after the spindle has been adjusted to the changing position. A drawback of this known positioning mechanism is the excessive manufacturing effort due to the fine-machined cardioid curve and the necessary actuating drive for the plunger. Moreover, due to the quill which carries the cardioid curve there is an increase of the rotating mass and of the outer diameter of the spindle The required arrangement of the plunger and its actuating drive adjacent the spindle results in an undesirable broadening of the spindle housing.